If the seeds are the part of the plant you’re interested in, it’s best to harvest them when they are fully ripe – any time from early autumn until the first frosts.
They won’t need drying, just storing in a cool dry place away from direct sunlight, like all your herbs.
Some seeds, like fennel, caraway, cardamon or aniseed, make lovely herb teas, especially if you bruise them first, and let them infuse for 4-5 minutes to bring out the full flavour. All of these help digestion and dispel wind. Others, like evening primrose, hemp, sunflower and sesame, are mainly valued for their oils. These will be broken down by heating, so the seeds are best added directly to food, or ground just before use.
You can also buy the oils, either liquid or in capsules, to take or to use in massage, creams and so on. And some seeds have their own particular properties. Pumpkin seeds have a high level of zinc, which helps to bring down inflammation, and they are particularly useful in the management of prostate problems. Celery seed helps to eliminate uric acid via the kidneys, which is useful to people who are prone to gout. And some seeds, like linseed and psyllium (flea seed) contain plenty of mucilage.
If you soak them in water, they swell and soften, and when you take them they act as a gentle bulk laxative, easing constipation and irritable bowel.
It’s the crossover with food that’s the most interesting thing. A seed is a compact package of life force, full of the nutrients the new plant needs to make a good start. Eating seeds, as they come or sprouted for a few days, makes those nutrients and that life force available to us. We eat them all the time, of course, in the form of our staple cereals like rice, wheat and so on, but the less obvious ones have a great deal more to offer.